1. Field of the Invention
This invention relates to a process for the production of solid alkali metal salts of .alpha.-sulfofatty acid alkyl esters containing less than 10% water, which are suitable for use as detergents in washing and cleaning preparations, by simultaneous bleaching and neutralization.
2. Statement of Related Art
The production of alkali metal salts of .alpha.-sulfofatty acid alkyl esters ("ester sulfonates") by sulfonation of fatty acid methyl esters with gaseous SO.sub.3 and subsequent neutralization of the sulfonic acids formed with aqueous alkali metal hydroxides, for example with sodium hydroxide, has been known for some time, The end products of this process, i.e. the alkali metal salts of the corresponding .alpha.-sulfofatty acid alkyl esters, are predominantly used as detergents in washing and cleaning preparations.
However, all hitherto known production processes have remained unsatisfactory either because, although light colored end products suitable for direct use in the detergent industry can be obtained, the yields of the sulfonation step leading to those products are highly unsatisfactory, or because although high sulfonation yields can be obtained, the color instability of the fatty acids or fatty acid esters in the sulfonation step means that dark-colored to brown-black crude products unsuitable for direct use in washing and cleaning preparations are regularly obtained. Accordingly, a sulfonation step taken to high product yields normally has to be followed by bleaching of the dark .alpha.-sulfofatty acid derivatives formed in the sulfonation step to obtain light-colored products suitable for use in washing and cleaning systems.
Another difficulty of hitherto known process for the production of aqueous ester sulfonates is that, in the course of the production process, the products accumulate in the form of water-containing pastes having an active-substance content of up to 70%. However, pastes such as these can no longer be pumped on their own, i.e. in the absence of further additives, above ester sulfonate concentrations of from 30% to 40%. Accordingly, the viscosity behavior of ester sulfonate pastes such as these always involves the risk of blockages in apparatus and pipes. Accordingly, there has long been a need to find production processes for alkali metal salts of .alpha.-sulfofatty acid alkyl esters in which which the high viscosity of the products obtained is harmless to the process.
CA vol. 101(1984)56874j (Japanese published patent application No. 84-16870) describes a process for the production of .alpha.-sulfofatty acid ester salts, in which .alpha.-sulfofatty acid esters formed during the sulfonation step are bleached with H.sub.2 O.sub.2 in the presence of a polycarboxylic acid or one of its salts and subsequently converted by neutralization into the corresponding .alpha.-sulfofatty acid esters salt. Although the salts formed are very suitable for use as detergents in washing and cleaning preparations, the reaction time required for the bleaching step is of the order of 1 hour which does not include the time required for the following neutralization reaction.
U.S. Pat. No. 4,547,318 describes a process for the production of color-stable, light-colored aqueous salt pastes of washing-active .alpha.-sulfofatty acid esters in which the dark-colored .alpha.-sulfofatty acid esters emanating from the sulfonation step are first prebleached with alkali metal hyperchlorites in neutral to mildly alkaline aqueous solution and are then fully bleached with hydrogen peroxide in the usual way in a mildly acidic aqueous solution. The bleaching process as a whole takes several hours, involves a change in the pH value of the reaction medium with all the ensuing disadvantages and gives aqueous salt pastes of which the active substance content is in the range of from 20 to 60% by weight which, as described above, gives rise to problems affecting the process due to the considerable increase in viscosity with increasing active substance content.